1. Field of the Invention
The present invention relates to an optical tomograph that obtains optical tomographic images by OCT (Optical Coherence Tomography) measurement.
2. Description of the Related Art
Conventionally, optical tomographs that utilize OCT measurement are employed to obtain tomographic images of living tissue. In an optical tomograph, a low coherence light beam emitted from a light source is divided into a measuring light beam and a reference light beam. Thereafter, a reflected light beam, which is the measuring light beam reflected by a measurement target when the measuring light beam is irradiated onto the measurement target, is combined with the reference light beam. Tomographic images are obtained, based on the intensity of a interference light beam obtained by combining the reflected light beam and the reference light beam (refer to U.S. Pat. Nos. 6,564,089 and 6,615,072, and Japanese Unexamined Patent Publication No. 2001-264246, for example).
OCT measurement can be roughly divided into two types, TD-OCT (Time Domain Optical Coherence Tomography) and FD-OCT (Fourier Domain Optical Coherence Tomography). In TD-OCT measurement, the intensity of the interference light beam is measured while changing the optical path length of the reference light beam. Thereby, intensity distributions of the reflected light beam corresponding to measuring positions in the depth direction of the measurement target (hereinafter, referred to as “depth positions”) are obtained.
On the other hand, in FD-OCT measurement, the optical path lengths of the reference light beam and the signal light beam are not changed. The intensity of the interference light beam is measured for each spectral component thereof, and frequency analysis, such as Fourier transform, is administered on the obtained spectral interference intensity signals. Thereby, intensity distributions of the reflected light beam corresponding to the depth positions of the measurement target are obtained. FD-OCT measurement has been gathering attention recently as a method that enables high speed measurement, due to the mechanical scanning associated with TD-OCT measurement being obviated.
Optical tomographs that perform SD-OCT (Spectral Domain Optical Coherence Tomography) measurement and optical tomographs that perform SS-OCT (Swept Source Optical Coherence Tomography) measurement are two types of optical tomographs that employ FD-OCT measurement. In an SD-OCT optical tomograph, a wide band low coherence light beam is emitted from an SLD (Super Luminescent Diode), an ASE (Amplified Spontaneous Emission) light source, or a white light source. The wide band low coherence light beam is divided into a measuring light beam and a reference light beam by a Michelson interferometer or the like. Thereafter, the measuring light beam is irradiated onto a measurement target, and a reflected light beam reflected by the measurement target is caused to interfere with the reference light beam. The interference light beam formed thereby is spectrally decomposed into each frequency component by a spectrometer, and the intensity of each frequency component of the interference light beam is measured by a detector array, in which elements such as photodiodes are provided in an array. A computer administers Fourier transform on the obtained spectral interference intensity signals, to obtain a tomographic image.
Meanwhile, an SS-OCT optical tomographs utilizes a light source that periodically sweeps the frequency of a laser beam. Reflected light beams of each wavelength are caused to interfere with reference light beams of each wavelength. Temporal waveforms of signals corresponding to the temporal variations in the frequency of the laser beam are measured, and a computer administers Fourier transform on the obtained spectral interference intensity signals, to obtain a tomographic image.
It is common for the various types of optical tomographs described above to employ lasers as light sources, and to employ optical fibers as optical waveguide means. Particularly when the optical tomographs are applied to endoscopes, it is common for optical fibers to be employed to guide light into body cavities.
During insertion of optical fibers into body cavities and during measurement, bending and twisting of the optical fibers occurs, as well as temperature changes due to insertion into the body cavities. Single mode fibers, which are generally employed in endoscopes, cannot preserve polarized states during propagation of light therethrough. Therefore, when forces such as bending and twisting, or variation factors such as temperature change and vibration are applied, the birefringent properties of the optical fibers varies irregularly.
In OCT measurement, an interference light beams is generated by combining a reflected light beam with a reference light beam. The intensity of the interference light beam varies according to the polarization directions of the reflected light beam and the reference light beam. The intensity of the interference light beam becomes maximal when the polarization directions of the reflected light beam and the reference light beam match. As described above, however, if the birefringent properties of the optical fibers varies, the polarization direction of the reflected light beam or the reference light beam also varies, resulting in variations in the intensity of the interference light beam. These intensity variations become fluctuations in interference signal levels, which causes density stripes to be generated in a tomographic image, thereby decreasing the image quality thereof. Depending on the degree of image quality deterioration, targets of diagnosis which should be discriminated may be overlooked.
The optical tomographs disclosed in U.S. Pat. Nos. 6,564,089 and 6,615,072 are provided with Faraday rotators at the tips of probes thereof, to compensate for variations in interference intensity due to birefringent changes caused by bending of an optical fiber. Magnetic garnet monocrystals, in which the crystal itself has magnetism, are employed as the material of the Faraday rotator. However, this type of Faraday rotator is likely to generate ghosts due to reflection, because the refractive index of the magnetic body is high. For this reason, reflection preventing measures, such as provision of a watertight seal filled with index matching fluid, forming bonding surfaces at angles other than right angles to prevent feedback of reflected light, and the like, become necessary. The reflection preventing measures lead to increases in manufacturing costs.
In addition, U.S. Pat. Nos. 6,564,089 and 6,615,072 disclose that it is important to employ polarization controllers that match the polarization direction of a light beam reflected by a measurement target and the polarization direction of a reference light beam, to cause the intensity of a interference light beam formed by interference of these two light beams to become maximal. However, in tomographs that employ the aforementioned polarization controllers, operating speeds are slow, because the polarization controllers are mechanically driven. Another shortcoming is that it takes time to find optimal combinations of operational parameters, as there are three parameters to be adjusted. Regarding adjustment by polarization controllers, there is the aforementioned problem of control speed thereof. Therefore, if the polarization direction shifts greatly during diagnosis utilizing OCT measurement, there is a possibility that diagnosis will be interrupted.